Recent reviews of studies that included both diet and exercise suggest that the combined interventions produce an approximately 2-kg greater weight loss compared to diet alone (King and Tribble, 1991). For example, Wood and colleagues (1991) studied the effect of a 1-year diet and activity intervention on weight in modestly overweight men and women. The interventions included diet alone and diet plus exercise. The men and women in the latter group sustained greater reductions of body weight, fat weight, and waist-to-hip ratio than those in the diet-only group. Recently, Bouchard et al. (1994) demonstrated in men that exercise is an efficient way to lose weight if subjects do not increase their food intake. The amount of energy necessary to maintain a stable body weight was determined in a group of young men isolated in a forestry camp. When the men consumed this amount of energy each day over 93 days but expended an additional 1,000 kcal/day in physical activity, they lost approximately 75 percent of the amount of weight predicted from the calculated energy deficit.

Unfortunately, energy intake and expenditure are difficult to measure with precision, which complicates assessment of energy balance in free-living individuals (see Appendix A). Consequently, several published studies show no relation of baseline energy intake (as estimated by a dietary assessment) or energy expenditure (as estimated by physical activity questionnaires) to subsequent weight change (Haffner et al., 1991; Klesges et al., 1992; Williamson et al., 1993). Some population-based studies show that increases in physical activity are associated with weight loss and decreases with weight gain (Klesges et al., 1992; Williamson et al., 1993). Interpreting these results is complicated by study designs in which activity and weight were measured at the same time, making it difficult to establish the temporal sequence of the changes. One recent report includes data from 530 women and 3,736 men who received three clinical examinations (Blair, 1993b). The average interval between the first and second examination was approximately 1.5 years, and the average interval between the second and third examination was approximately 4.5 years. Increases in fitness (assessed by a maximal exercise test) during the first interval were associated with decreases in body weight and skinfolds by the time of the third examination. Declines in fitness and reductions in self-reported physical activity from the first to the second examination were associated with weight gain at the third examination. These results held after multivariate adjustment for age and smoking habits.

Body-fat stores accumulate only as a result of a positive energy balance, and obesity usually develops from a small positive energy balance over the long term. Although programs may focus on diet or physical activity alone, emphasis should be placed on both energy expenditure and energy intake.